Turbochargers can increase the power of engines by providing additional air to the engine cylinders. An exhaust-gas driven turbine connected to a compressor may be used to produce the additional air. However, turbocharger lag, which occurs while turbocharger turbines develop adequate rotational speed, can be a problem. One method for reducing turbocharger lag is to decrease the weight of the turbocharger's rotating parts, including the turbine and the shaft attached to the turbine.
Titanium-aluminide constitutes a lightweight, strong material that may be used to produce turbocharger turbines. However, the use of titanium-aluminide can complicate joining of the turbine to the turbocharger shaft, which is often made with steel. Titanium-aluminide and steel have different thermal expansion properties and may produce undesirable phase transformations at their material interfaces. Therefore, because turbochargers experience significant temperature variations, a titanium-aluminide turbine and a steel shaft may be unsuitable for joining directly to one another.
One method of joining titanium-aluminide turbines to steel shafts is disclosed in U.S. Pat. No. 6,291,086. The method describes the use of an interlayer material disposed between a titanium-aluminide turbine and steel shaft. The interlayer material is welded to both the titanium-aluminide turbine and steel shaft. Therefore, although this method may provide a suitable connection between the turbine and shaft, two welds must be made and an additional material must be used, which can add significant time and cost to production. Further, use of a steel shaft adds significant weight to the turbocharger, which can increase turbocharger lag. Thus, turbocharger shafts fabricated from lighter materials such as titanium or titanium alloys are preferable as indicated in U.S. Patent Application Publication 2006/0067824, which discloses titanium aluminide turbine boned to a titanium shaft by various means such as gas tungsten-arc welding, gas metal-arc welding, resistance welding, laser welding, plasma arc welding, electron-beam welding, friction welding, brazing, and soldering.
However, bonding a turbine made from a titanium aluminide to a shaft made from a titanium alloy is challenging because of three reasons: high local thermal stress involved with bonding process; formation of brittle intermetallic phases at the bonding interface; and inherent low room temperature ductility of titanium aluminide alloys. Because of these reasons, the bonding interface between a titanium aluminide turbine and a titanium alloy shafts and even the titanium aluminide turbine itself are prone to crack during or after the bonding process.
In some applications, because of specific geometry or a large size of the titanium aluminide turbine, the local thermal stresses can become extremely high and therefore render the bonding process even more challenging. For example, in turbine rotor applications, the bonding interface is fairly close to rear face of the turbine, and the geometry of the turbine hub changes rapidly. This rapid change in geometry, in addition to the large thermal mass of turbine wheel, may cause a steep temperature gradient, and therefore, may cause large thermal stress which may exceed the strength of the titanium aluminide in the vicinity of the bonding interface with the titanium alloy shaft.
One method of bonding a titanium aluminide component to a titanium alloy component involves the use of friction welding as disclosed in US2008/0000558. The titanium aluminide component is heated to a temperature between 300° C. and 800° C. The titanium alloy component is rotated relative to the titanium aluminide component. The titanium aluminide and titanium alloy components are pressed against each other while the titanium alloy component is rotated. The rotation is then stopped, and the two components are pressed against each other again as a forging step after the rotation is stopped.
However, despite these recent advances, the joint between a titanium aluminide turbine and a titanium alloy shaft may crack during or after the joining process. The friction welding process imposes substantial thermal stresses. Because titanium aluminide and other intermetallic phases formed at the welding interface have a limited ductility, such components or joints can be prone to crack under stress. Further, the stresses are often increased because of the geometry of the turbine and shaft and the size of the two components. Thus, the joining of a titanium aluminide turbine to a titanium alloy shaft is very challenging.